A scenario is a coherent, internally consistent, and plausible description
of a possible future state of the world. Scenarios are commonly required in
climate change impact, adaptation, and vulnerability assessments to provide
alternative views of future conditions considered likely to influence a given
system or activity. A distinction is made between climate scenarios, which describe
the forcing factor of focal interest to the IPCC, and nonclimatic scenarios,
which provide the socioeconomic and environmental context within which climate
forcing operates. Most assessments of the impacts of future climate change are
based on results from impact models that rely on quantitative climate and nonclimatic
scenarios as inputs. [3.1.1, Box
3-1]

3.2. Socioeconomic, Land-Use, and Environmental Scenarios

Nonclimatic scenarios describing future socioeconomic, land-use, and environmental
changes are important for characterizing the sensitivity of systems to climate
change, their vulnerability, and the capacity for adaptation. Such scenarios
only recently have been widely adopted in impact assessments alongside climate
scenarios.

Socioeconomic scenarios. Socioeconomic scenarios have been used more
extensively for projecting GHG emissions than for assessing climate vulnerability
and adaptive capacity. Most socioeconomic scenarios identify several different
topics or domains, such as population or economic activity, as well as background
factors such as the structure of governance, social values, and patterns of
technological change. Scenarios make it possible to establish baseline socioeconomic
vulnerability, pre-climate change; determine climate change impacts; and assess
post-adaptation vulnerability. [3.2]

Land-use and land-cover change scenarios. Land-use change and land-cover
change (LUC-LCC) involve several processes that are central to the estimation
of climate change and its impacts. First, LUC-LCC influences carbon fluxes and
GHG emissions, which directly alter atmospheric composition and radiative forcing
properties. Second, LUC-LCC modifies land-surface characteristics and, indirectly,
climatic processes. Third, land-cover modification and conversion may alter
the properties of ecosystems and their vulnerability to climate change. Finally,
several options and strategies for mitigating GHG emissions involve land cover
and changed land-use practices. A great diversity of LUC-LCC scenarios have
been constructed. Most of these scenarios do not address climate change issues
explicitly, however; they focus on other issuesfor example, food security
and carbon cycling. Large improvements have been made since the SAR in defining
current and historic land-use and land-cover patterns, as well as in estimating
future scenarios. Integrated assessment models currently are the most appropriate
tools for developing LUC-LCC scenarios. [3.3.1, 3.3.2]

Environmental scenarios. Environmental scenarios refer to changes in
environmental factors other than climate that will occur in the future regardless
of climate change. Because these factors could have important roles in modifying
the impacts of future climate change, scenarios are required to portray possible
future environmental conditions such as atmospheric composition [e.g., carbon
dioxide (CO2), tropospheric ozone, acidifying compounds, and ultraviolet-B
(UV-B) radiation]; water availability, use, and quality; and marine pollution.
Apart from the direct effects of CO2 enrichment, changes in other
environmental factors rarely have been considered alongside climate changes
in past impact assessments, although their use is increasing with the emergence
of integrated assessment methods. [3.4.1]